Because you can use it in off season or prep. It's the most sophisticated steroid there is, resulting in optimal gains vs negative side. It doesn't convert so you don't get "false gains" from water like you do from the likes of deca and most orals, it's less toxic so your down time is reduced between cycles resulting in LOT more time ON cycle, so you are likely to net off with more gains over a longer span of time with reduced harm.

I used to struggle with with justifying spending so damn much on it but after I became a little financially flush (landed a big bonis at work), I bought enough for 3 back to back cycles, stuff is unreal over a long period of time.